1. Field of the Invention
The present invention relates to stereo broadcasting receiving devices for receiving stereo broadcasting such as FM broadcasting.
2. Description of Prior Art
In FM broadcasting, an L+R signal (a main channel signal) obtained by adding a left stereo signal and a right stereo signal and an Lxe2x88x92R signal (a sub-channel signal) obtained by subtracting the right stereo signal from the left stereo signal generally constitute a broadcasting signal. Specifically, a composite wave signal having an L+R signal, a double sideband signal obtained by amplitude-modulating a sub-carrier (38 KHz) using an L-R signal and suppressing a carrier, and a pilot signal arranged, respectively, at a frequency of 50 to 15000 Hz, a frequency of 23 to 53 KHz, and a frequency of 19 KHz is taken as a broadcasting signal.
In order to take out a left stereo output signal (an L signal) and a right stereo output signal (an R signal) from the broadcasting signal, addition and subtraction of the L+R signal and the Lxe2x88x92R signal may be performed.
When frequency modulation (FM modulation) is performed, so-called triangular noise, centered around the frequency of the carrier is produced. The higher the frequency is, the larger the triangular noise is. When the receiving level of the broadcasting signal is low, therefore, the Lxe2x88x92R signal conveyed at a frequency higher than that of the pilot signal must be attenuated. When the Lxe2x88x92R signal is attenuated, however, the degree of separation between the right and left stereo signals is decreased. That is, separation of the L and R signals is performed by addition and subtraction of the L+R signal and the Lxe2x88x92R signal, i.e., (L+R)+(Lxe2x88x92R)=2L and (L+R)xe2x88x92(Lxe2x88x92R)=2R. When the Lxe2x88x92R signal is attenuated, therefore, the R signal leaks out to the L signal, while the L signal leaks out to the R signal, so that the degree of separation is decreased.
JP-A-6-315016 discloses a noise reducing circuit for preventing the degree of separation of right and left stereo signals from being decreased when the receiving state gets worse.
The noise reducing circuit described in JP-A-6-315016 comprises a stereo separating circuit for performing addition and subtraction of an L+R signal and an Lxe2x88x92R signal to take out an L signal and an R signal, an attenuating circuit for attenuating the Lxe2x88x92R signal, a storage for storing digital data representing the relationship between the receiving level of a desired station signal and the amount of attenuation of the Lxe2x88x92R signal, a desired station signal level detection circuit for detecting the receiving level of the desired station signal, and control means for reading out the data stored in the storage on the basis of the detected receiving level of the desired station signal and controlling the amount of attenuation in the attenuating circuit on the basis of the data read out.
In the invention described in JP-A-6-315016, when the receiving state of a broadcasting signal is bad, however, the Lxe2x88x92R signal is attenuated to improve the S/N ratio. Accordingly, the L signal and the R signal cannot be separated from each other and taken out, and only the demodulation of a monophonic signal can be realized. Particularly in an on-vehicle FM receiving device, the receiving state may get worse in many cases as the vehicle travels. In such a case, a sound field having a broadening feeling cannot be reproduced, which does not give listeners entire satisfaction.
An object of the present invention is to provide a stereo broadcasting receiving device capable of ensuring stereo broadcasting even if the receiving state is bad and reproducing a sound field having a broadening feeling.
A first stereo broadcasting receiving device according to the present invention is characterized by comprising receiving means for receiving a broadcasting signal having an L+R signal obtained by adding a left stereo signal and a right stereo signal and an Lxe2x88x92R signal obtained by subtracting the right stereo signal from the left stereo signal; separation means for separating the broadcasting signal received by the receiving means into the L+R signal and the Lxe2x88x92R signal; pseudo stereo signal generation means for generating a pseudo left stereo signal and a pseudo right stereo signal from the L+R signal obtained by the separation in the separation means; first multiplication means for multiplying the pseudo left stereo signal by a first coefficient; second multiplication means for multiplying the pseudo right stereo signal by the first coefficient; third multiplication means for multiplying the L+R signal obtained by the separation in the separation means by a second coefficient; fourth multiplication means for multiplying the Lxe2x88x92R signal obtained by the separation in the separation means by a third coefficient; receiving state judgment means for judging the receiving state of the broadcasting signal received by the receiving means; coefficient determination means for determining the first coefficient, the second coefficient, and the third coefficient on the basis of the result of the judgment in the receiving state judgment means; means for generating a left stereo output signal on the basis of an output signal of the first multiplication means, an output signal of the third multiplication means, and an output signal of the fourth multiplication means; and means for generating a right stereo output signal on the basis of an output signal of the second multiplication means, the output signal of the third multiplication means, and the output signal of the fourth multiplication means.
The coefficient determination means determines the ratio of the first coefficient, the second coefficient, and the third coefficient to be n (n greater than 0):0:0 when the receiving state judgment means judges that the receiving state is bad.
A second stereo broadcasting receiving device according to the present invention is characterized by comprising receiving means for receiving a broadcasting signal having an L+R signal obtained by adding a left stereo signal and a right stereo signal and an Lxe2x88x92R signal obtained by subtracting the right stereo signal from the left stereo signal; separation means for separating the broadcasting signal received by the receiving means into the L+R signal and the Lxe2x88x92R signal; first multiplication means for multiplying the L+R signal obtained by the separation in the separation means by a first coefficient; second multiplication means for multiplying the L+R signal obtained by the separation in the separation means by a second coefficient; third multiplication means for multiplying the Lxe2x88x92R signal obtained by the separation in the separation means by a third coefficient; pseudo stereo signal generation means for generating from an output signal of the first multiplication means a pseudo stereo signal comprising a pseudo left stereo signal and a pseudo right stereo signal; receiving state judgment means for judging the receiving state of the broadcasting signal received by the receiving means; coefficient determination means for determining the first coefficient, the second coefficient, and the third coefficient on the basis of the result of the judgment in the receiving state judgment means; means for generating a left stereo output signal on the basis of the pseudo left stereo signal, an output signal of the second multiplication means, and the output signal of the third multiplication means; and means for generating a right stereo output signal on the basis of the pseudo right stereo signal, an output signal of the second multiplication means, and the output signal of the third multiplication means.
The coefficient determination means determines the ratio of the first coefficient, the second coefficient, and the third coefficient to be n (n greater than 0):0:0 when the receiving state judgment means judges that the receiving state is bad.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.